NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Douglas DeVoto

Principal Investigator

National Renewable Energy Laboratory

Annual Merit Review and Peer Evaluation Meeting

June 18, 2014

Washington, D.C.

PR-5400-61358

This presentation does not contain any proprietary, confidential, or otherwise restricted information.

Reliability of Electrical 

Interconnects

Project ID: APE036
2

Overview

Timeline

• Project Start Date: FY11

• Project End Date: FY14

• Percent Complete: 80%

Barriers and Targets

• Efficiency

• Performance and Lifetime

Budget

• Total Project Funding:

DOE Share: $1,600K

• Funding Received in FY13: 

$450K

• Funding for FY14: $300K

Partners

• Interactions/Collaborations

o Curamik, Kulicke and Soffa

• Project Lead

o National Renewable Energy 

Laboratory
3

Relevance

Ribbon Bonding

400 

μm

2,000 μm x 200 μm

400 

μm

400 

μm

Three 400-μm wires can be 

replaced by a single 2,000-μm x 

200-μm ribbon for equivalent 

current carrying capability.

Traditional Power Electronics Package

Wire Bonding

IGBT/Diode Die

Metalized Substrate

Substrate

Attach

Base Plate

Die Attach

Interconnect Encapsulant

Enclosure

Terminal
4

Relevance

• Technology Benefits

o A transition from round wire interconnects to ribbon interconnects 

allows for higher current densities, lower inductances, and lower loop 

heights.

• Overall Objective

o Identify failure modes in ribbon bonds, experimentally characterize their 

life under known conditions, and develop and validate physics-of-failure 

(PoF) models that predict life under use conditions.

o Test and model ribbon bonds to prove they exhibit equivalent or greater 

reliability than industry-accepted wire bond technology.

• Uniqueness and Impacts

o Failure modes and PoF models for emerging interconnect technology.
5

Milestones

2013 2014

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

Fabricate samples at Kulicke and Soffa for 

51 additional test substrates.

Measure baseline 

ribbon bond 

strength.

Conduct accelerated life testing plan.

Validate lifetime estimation 

models for specific failure modes 

observed in accelerated tests.

Go/ 

No-Go

Key 

Deliverable

Go/No-Go: Ribbon bonds must meet minimum pull strength before proceeding with 

accelerated tests.

Key Deliverable: Report on the reliability of ribbon bond technology to industry.
6

Overview

Sample Synthesis

Ribbon Bonding

Accelerated Testing

Temperature 

Elevation

Temperature 

Cycling

Power Cycling

Corrosion Testing

Vibration Testing

Pull Testing

Sample Evaluation

Lifetime 

Estimation

Model Validation

Approach

Bond Pad 

Optimization
7

Sample Synthesis

Criterion Variation

Bonding Material Al 

Ribbon

Cu/AlClad 

Ribbon

Al Wire

Cross Section (μm)

2,000 x

200 

Ribbon

1,000 x

100 

Ribbon

300⌀

Wire

Ribbon Span (mm) 10 20

Tool Pattern Waffle Three-Ridge

Ribbon Stacking Not Stacked Stacked

Forced Bond Angle (°) 0 20

Bonding Power Level Low High

Bond Pad Interface Cu Si (Al)

Ribbon Material

Ribbon Span

Ribbon Cross Section

Tool Pattern

Stacked Pads Forced Bond Angle

Approach
8

Accelerated Testing Plan

Accelerated Test Testing Condition Duration Standard

Initial Pull Test - - -

Temperature Elevation 150°C 500/1,000 hours JESD22-A103D

Temperature Cycling -40°C to 150°C, less than 20 second transition time 1,500/3,000 cycles JESD22-A104D

Corrosion Testing

130°C, 85% relative humidity 96 hours JESD22-A110D

110°C, 85% relative humidity 264 hours JESD22-A110D

121°C, 100% relative humidity 96 hours JESD22-102D

Power Cycling 40°C to 120°C, ~ 2 second cycled DC bias 3,000 cycles JESD22-A105C

Vibration Testing Combined vibration and thermal cycling Until interconnect 

fails HALT

Approach

Temperature

Vibration

Temperature

+ Vibration

Shear Stress

Flexural

Stress Corrosion

Intermetallics

CTE (x 10-6/K)

Silicon: 2.6

Copper: 16.5

Aluminum: 22.7

20

40

60

80

100

0 2 4 6 8

Temperature (°C)

Time (s)

Power Cycling

Thermal Cycling
9

Sample Evaluation

1

2

3

4

5

Failure Modes

1: Wire/ribbon break

2: Heel failure – substrate

3: Heel failure – die

4: Bond lift-off – substrate

5: Bond lift-off – die

• Ribbon pull testing indicates the strength of the ribbon bond.

• Bond strength and failure mode is recorded for each bond.

Approach
10

Sample Evaluation

Failure Mode: Wire Break

Approach
11

Sample Evaluation

Failure Mode: Heel Failure from Substrate

Approach
12

Sample Evaluation

Failure Mode: Bond Pad Lift-off from Substrate

Approach
13

Sample Evaluation

• The minimum bond strength is specified by MIL-STD-883H Method 

2011.8.

o Minimum bond strength requirement increases with respect to 

increasing interconnect cross-sectional area. 

1,000 x 100 μm, 1.28 N

500 μm, 2.13 N

2,000 x 200 μm, 3.50 N

300 μm, 0.96 N

0.01

0.1

1

10

10 100 1,000

Minimum Bond Pull Limit (N)

Equivalent Wire Diameter (μm)

Minimum Bond Strength

Approach
14

Sample Test Substrates

• 51 test boards bonded at Kulicke and Soffa.

o 48 ribbons bonded per board in 12 parallel electrical paths.

o Loop height to span ratio is 1:2.2.

Ribbon Bonding Test Board Layout

10 mm span, 0° angle, 8x

20 mm span, 20° angle, stacked, 3x 20 mm span, 0° angle, 7x

20 mm span, 20° angle, 6x

Low Power Level High Power Level

Technical Accomplishments 
15

Bond Optimization

10

14

18

22

26

1,000 1,200 1,400 1,600 1,800

Deformation (

μm)

Current (mA)

Current 

(mA)

Deformation 

(μm)

Deformation 

(visual)

1,070 11

1,210 15

1,370 19

1,490 21

1,630 23

• Ultrasonic bonding power, force, and 

application time contribute to the quality of 

the bond pad.

• Bond quality is measured by:

o Bond pad deformation

o Pull strength

o Failure mode.

• Deformation pattern depth increases with 

increasing current levels.

Technical Accomplishments 
16

Bond Optimization

• Pull strength of test bonds was 

measured to be approximately 9.8 N.

• Pull failure modes varied with bond 

current levels.

o These criteria were used for 

final selection of optimized 

bonding current level.

1

2

3

4

5

Failure Modes

1: Wire/ribbon break

2: Heel failure – substrate

3: Heel failure – die

4: Bond lift-off – substrate

5: Bond lift-off – die

0%

25%

50%

75%

100%

1,070 1,210 1,370 1,490 1,630

Failure Mode Percentage

Bonding Current Level (mA)

1: Wire/ribbon break

2: Heel failure – substrate

3: Heel failure – die

4: Bond lift-off – substrate

5: Bond lift-off – die

Technical Accomplishments 
17

Baseline Evaluation

• Initial pull testing was completed on test substrates prior to accelerated 

testing:

o Al wire has a cross-section of 500 μm.

o Ribbon interconnects have 1,000 μm x 100 μm cross-sections.

o Bonding power for ribbon interconnects is specified as either low or 

high.

Technical Accomplishments 

0

5

10

15

20

25

30

Pull Force (N)

Interconnect Material

10 mm

20 mm

Al Wire Cu/Al Ribbon (low) Cu/Al Ribbon (high) Al Ribbon (low) Al Ribbon (high)
18

0%

25%

50%

75%

100%

Al Wire Cu/Al Ribbon (low) Cu/Al Ribbon (high) Al Ribbon (low) Al Ribbon (high)

Failure Mode Percentage

Bonding Method

Wire/ribbon break

Heel failure

Bond lift-off

Baseline Evaluation

• The failure mode was recorded for each 

bond prior to accelerated testing:

o Al wire bonds and Cu/Al-clad 

ribbon failures showed bond lift-off 

failures.

o Al ribbon bonds exhibited heel 

failures.

1

2

2

3

3

Failure Modes

1: Wire/ribbon break

2: Heel failure

3: Bond lift-off

Technical Accomplishments 
19

Accelerated Test Testing Condition Duration

Temperature Elevation 150°C 500/1,000 hours

Post-Accelerated Testing EvaluationTechnical Accomplishments 

6

7

8

9

10

11

12

Pull Force (N)

Testing Time

Al Ribbon (low) Temperature Elevation 10 mm

20 mm

Initial 500 hours 1,000 hours

4

5

6

7

8

9

10

Pull Force (N)

Testing Time

Al Ribbon (high) Temperature Elevation 10 mm

20 mm

Initial 500 hours 1,000 hours
20

Temperature Elevation Evaluation

• The failure mode was recorded for each 

bond after temperature elevation 

testing:

o Al ribbon heel failure mode 

remained the same through 

temperature elevation testing.

1

2

2

3

3

Failure Modes

1: Wire/ribbon break

2: Heel failure

3: Bond lift off

Technical Accomplishments 

0%

25%

50%

75%

100%

Initial 500 hours 1,000 hours

Failure Mode Percentage

Testing Time

Al Ribbon (low) Temperature Elevation

Wire/ribbon break

Heel failure

Bond lift-off

0%

25%

50%

75%

100%

Initial 500 hours 1,000 hours

Failure Mode Percentage

Testing Time

Al Ribbon (high) Temperature Elevation

Wire/ribbon break

Heel failure

Bond lift-off
21

Responses to Previous Year Reviewers’ Comments

The reviewer requested that the summary provide 

general observations and conclusions.

Knowledge transfer of failure modes to industry is 

a key milestone of this project.

... it was unclear to the reviewer how ultrasonic 

ribbon bonding labor and new equipment costs 

compares to standard wire bonding.

In many cases, wire bonding equipment can be 

retrofitted to add ribbon bonding capability.

The reviewer suggested that in the future an EV 

component manufacturer might be added to the 

collaborators.

Knowledge transfer to manufacturers will be 

accomplished through industry visits.
22

Collaboration and Coordination

• Partners

o Curamik (Industry): technical partner on substrate design

o Kulicke and Soffa (Industry): technical partner on wire and ribbon 

bonding procedure
23

Remaining Challenges and Barriers

• The design-of-experiments required to cover 

all combinations is large:

o Strategically choosing key experiments reduces 

the overall set of combinations.

o Sought experience from the technical community 

to guide our choices in experimental tests and 

ribbon bonding geometries.
24

Proposed Future Work (FY14)

• Complete thermal, power, and environmental testing on 

ribbon bonds.

• Report on mechanical reliability of ribbon bonds under 

testing, and make recommendations to industry partners.

• Validate lifetime estimation models for specific failure modes 

observed in accelerated tests.
25

Summary

• DOE Mission Support:

o Transitioning from wire bonding to ribbon bonding manufacturing will 

advance power electronics technology for compact, reliable packaging with 

higher current capabilities.

• Approach:

o Synthesize ribbon bonds with varying material (Al, Cu/Al) and geometry 

(cross section, span and loop height, pad length, number of stitches, stacked 

pads, and forced angles) parameters.

o Conduct comprehensive reliability testing, including temperature elevation, 

temperature cycling, power cycling, and corrosion testing.

o Revise wire bond models to be applicable to ribbon bonding.

• Accomplishments:

o Test samples were synthesized, and reliability testing was initiated.

o Initial accelerated tests and interconnect bond strength evaluations were 

completed.
26

Summary

• Collaborations:

o Curamik, Kulicke and Soffa

• Future Work:

o Complete thermal, power, and environmental testing on ribbon bonds.

o Report on mechanical reliability of ribbon bonds under testing and make 

recommendations to industry partners.

o Validate lifetime estimation models for specific failure modes observed in 

accelerated tests.
For more information, contact:

Principal Investigator

Douglas DeVoto

Douglas.DeVoto@nrel.gov

Phone: (303) 275-4256

APEEM Task Leader

Sreekant Narumanchi

Sreekant.Narumanchi@nrel.gov 

Phone: (303) 275-4062

Acknowledgments:

Susan Rogers and Steven Boyd, 

U.S. Department of Energy

Team Members:

Paul Paret

Tao Xu (K&S)